This invention relates to resilient compressible printing elements and in particular to those having an intermediate layer of foamed rubber.
It is known in producing resilient compressible printing elements to have a cellular intermediate layer as described in the article "New Developments in Off-Set Blankets" pages 2-7, Professional Printer, Volume 22, Number 6. However, the only closed cell materials revealed in the article were those made using microsheres which were crushed. When blowing agents were used an open cell structure was produced in which the cell walls ruptured causing the cells to be interconnected. In the article "New Development in Off-Set Blankets" at page 3 it is pointed out that open celled foams are not satisfactory while the closed cells produced by the breaking of microspheres yielded good results because the closed structure recovered more quickly than the open structure because the gas contained in the voids was compressed and only had to expand after compression. Among other deficiencies, the use of microspheres is an expensive manufacturing procedure and results in the retention of a substantial amount of residue within the void from the microsphere body. U.S. Pat. No. 3,887,750 shows the use of discrete hollow fibers to obtain a closed cell and U.S. Pat. No. 3,795,568 shows the use of particles of compressible latex foam rubber to obtain the closed cells. Both of these approaches have the disadvantage of having substantial internal structure within the closed cell of the matrix forming the compressible layer. They also require the premanufacture of the structures to be incorporated in the rubber matrix.
Closed cell systems enable the provision of sufficient unfoamed rubber surrounding the cells to provide a large enough tensile force to prevent delamination or internal splitting of the compressible foam layer. This property also permits the use of thicker compressible layers to provide adequate void volume to absorb minor smashes preventing damage to the printing blanket.
A closed celled system also prevents capillary absorption of solvent through the edges of a printing element. Open celled foam and non-woven compressible layers are subject to capillary absorption with resulting weakening of the compressible layer.
It is a primary object of the present invention to provide a better printing blanket, particularly for uses such as lithographic printing.
Another object of the present invention is to provide a printing blanket that does not require a fabric reinforcing between the compressible layer and the printing face.